1. Field of the Invention
The present invention relates to an EL semiconductor device having an n-type semiconductor layer structure on an InP substrate; and, more particularly, to an EL semiconductor device, e.g., a green light emitting laser, featuring a sufficient energy gap and carrier concentration, including an n-type cladding layer made out of II and VI group elements and capable of trapping and confining light and carriers.
2. Background of the Related Arts
An EL semiconductor device, e.g., a semiconductor laser diode (LD), is a light source used for recording or reproducing data onto or from an optical disc. Historically, most of the research has been focused on development of a light source with a shorter wavelength to be able to make higher recording densities in CDs, DVDs, or Blu-ray discs.
Besides some use in optical discs, LD of 1.55 μm, 1.3 μm, and 0.98 μm operating wavelengths for use in communications has been developed. In effect, its application range is gradually being expanded to other fields such as a solid laser excitation unit, processing, sensor, measuring instrument, medical treatment, display, etc. While a red LED has been put to practical use, blue and green-emitting superluminescent diodes have not yet been developed for years. In the meantime, a new AlGaInN-based device was developed and has been put to practical use, and later researches have continually progressed to ultraviolet or white as its application. In result, LED became an essential element in people's lives nowadays and is implemented in a broader application range than LD, not only for indicators but also for displays and illumination.
In case of green color having the highest visibility for human eye, a green LED is still low in efficiency compared with other color LEDs and no one has yet discovered a high efficiency LD in a visible ray having a wavelength within the range from about 480 nm (pure blue) to about 600 nm (orange). Therefore, if a green LD, green being one of three primary colors, can be developed, it certainly will open new application fields.
As for semiconductor materials for those optical devices, AlGa(In)As based materials were used for infrared devices with a wavelength of a 780 nm, 808 nm, 860 nm, 915 nm, or 980 nm band, and InGaAsP based Group III-V compounds were used for devices with a wavelength of 1.3 μm or 1.55 μm. In addition, with technical advances in researches, AlGaInP based Group III-V compound semiconductors and AlGaInN based Group III-V nitride semiconductors were developed and are now put to practical use for red light emitting devices with a wavelength of a 600 nm band (635-670 nm to be specific) and blue light emitting devices with a wavelength of a 400 nm band (350-480 nm to be specific), respectively.
For an unexplored region around 480-600 nm related to the technical field of the present invention, III-V group compound semiconductors as well as II-VI group semiconductors are leading candidates, and a cyan-blue LD around 500 nm has actually been reported to have an about 400-hour average lifetime under the conditions of room temperature and continuous wave lasing at 1 mW (E. Kato at al. “Electronics Lett.”, 34, (1998), p. 282). However, II-VI group semiconductors include Hg or Cd for example, which are strongly toxic substances to a human body. As a matter of fact, except for special cases, it is very hard to use these two substances as a product.
The European Union (EU) RoHS Directive was adopted in Jan. 27, 2003 (Official Gazette dated February 13) and came into force on 1 Jul. 2006, imposing the Restriction of the use of certain Hazardous Substances (RoHS) by which all electrical and electronic equipment manufacturers were banned from using 6 kinds of substances such as Pb, Hg, Cd, heavy metals with hexavalent chromium, and Brominated Flame Retardants (BFR) (polybromo diphenyl ether (PBDE) and polybromo biphenyl (PBB)) in their products by Jul. 1, 2006. Similarly, J-Moss (Japanese Ministerial Ordinances) was imposed which requires the same by Jul. 1, 2006.
Although details on the legislation are not provided here, it would be sufficient to mention that there were 17 examples of application restrictions and maximum allowances of the RoHS substances. Starting with manufactures playing a key role in the environmental activities for society, there were many who announced a complete stop of using those 6 kinds of RoHS substances, so it was pretty much impossible to apply those restricted substances to new products.
The inventors and several research groups in or outside of Japan noticed MgxZnyCd1-x-ySe Group II-VI compound semiconductors that can be manufactured via crystal growth and lattice-matched to an InP semiconductor substrate as one of material candidates for forming a semiconductor device generating yellow or green, and continued researches from there (refer to N. Dai et al., “Appl. Phys. Lett.” 66, (1995), p. 2742 and T. Morita et al., “J. Electron. Mater.” 25, (1996) p. 425). It is known that MgxZnyCd1-x-ySe can be lattice matched to InP if each composition (x, y) satisfies a relation of y=0.47−0.37x (x=0 to 0.8, y=0.47 to 0.17), and a forbidden band thereof can be controlled from 2.1 eV up to 3.6 eV by changing its composition from (x=0, y=0.47) to (x=0.8, y=0.17).
The above-described composition range give energy gaps of the direct transition type, and those energy gaps correspond to wavelengths between 590 nm (Mars yellow, orange color) and 344 nm (ultraviolet). This indicates that active and cladding layers to form a double hetero structure, which is the basic structure of a yellow or green light emitting semiconductor device, can be realized simply by changing the composition of MgxZnyCd1-x-ySe.
In fact, photoluminescence measurements of the MgxZnyCd1-x-ySe grown on an InP substrate by Molecular Beam Epitaxy (MBE) give good luminescence properties with a peak wavelength in a range from 571 nm to 397 nm for MgxZnyCd1-x-ySe with a different composition (T. Morita et al., “J. Electron. Mater.” 25, 425 (1996)).
Also, there was a report that laser oscillation by photo-excitation was observed in each of red, green and blue bands of an MgxZnyCd1-x-ySe laser structure (L. Zeng et al., “Appl. Phys. Lett.” 72, 3136 (1998)).
Meanwhile, there has been no report so far on laser oscillation by current driving in a semiconductor laser diode that is formed of MgxZnyCd1-x-ySe only. The main reason for the absence of laser oscillation is because it is difficult to control p-type conductivity by impurity doping of MgxZnyCd1-x-ySe.
Therefore, with an n-type MgxZnyCd1-x-ySe cladding layer, we found ideal materials for an active layer, p-cladding layer, guide layer, contact layer and the like to prepare a test.
For instance, we used MgZnCdSe for an n-cladding layer, ZnCdSe for an active layer, and MgSe/BeZnTe for a p-cladding layer, and succeeded to obtain a 560 nm yellow-green LD's oscillation at 77K. Further, using BeZnSeTe for an active layer, we observed a single peak producing Mars yellow (orange color) or yellowish green at 594 nm, 575 nm, and 542 nm. It turned out that a 575 nm LED lasts longer than 5000 hours at room temperature.
Particularly, a close examination on light emission mechanism in an LED including an n-cladding layer that is formed of MgxZnyCd1-x-ySe or MgSe/ZnCdSe superlattice structure and a BeZnSeTe active layer indicated emission wavelength is substantially influenced by driving current, and the type II luminescence can be observed at the hetero-interface in the vicinity of the n-cladding layer and the active layer.